If Washington can't / won't regulate gun sales. ... can banks?

Yeah, that's the market working. You can bet there'd be a regional bank that's not affiliated with Chase, Citi or BOA that would wedge into that niche and start offering 0 percent interest on the first $10,000 spent on AR-15s.

 

My only point of reference is the immediate aftermath of the ridiculous UIGEA.
I had a Visa CC/debit card through one bank that blocked me from online gambling, and I had another Visa CC/debit card that did not block me. The difference was the issuing bank, not the card network.

 

Google "credit card guns" and you'll get this:

https://nypost.com/2017/08/08/the-worlds-tiniest-gun-fits-in-your-wallet/

Unbelievable.

 

First National Bank of Omaha, a major issuer nationally of credit cards, said Thursday it would end its relationship with the National Rifle Association.


First National Bank ends relationship with NRA

 

I believe it was in 1996 when Sheryl Crow released a song referencing children shooting each other with a gun bought at Walmart. Instead of taking it as a sign they needed to consider changing policy, Walmart got pissed and stopped selling Sheryl Crow CDs. So yeah, I could totally see them putting pressure on banks, credit card companies and processors in this case.
It's an interesting idea overall, and anything limiting access to assault rifles is fine by me, whether this idea with cards or government legislation. But as others have pointed out, the credit card thing will just push transactions to the black market with cash.
Here in Oregon, we have recreational marijuana shops. I'm pretty sure they are cash only, and that doesn't seem to be deterring business. I can't recall exactly what checking is done to make sure the buyer is of age and regulating rules about where consumption can take place and all of that, but maybe that could be used as a model?

 

Three major car rental companies dump the NRA

Until Thursday, Enterprise Holdings, which operates Enterprise, Alamo, and National, had a partnership with the NRA to provide discounts to members of the gun lobby. The discounts were available to NRA members once they paid the $40 annual fee, and the companies were among the 22 corporations offering discounts and “five star savings” to the gun lobby’s members.

 

Not a bank, but yet another possible source of pressure on the gun manufacturers.

BlackRock puts gunmakers on notice after Florida school shooting

 

That's one company, with three brands.

 

ok

 

This might be the one way to fix the problem, given that politicians seem unwilling. Go at the NRA. Rob them of their money. Make a membership something shameful.